Unlike most mammalian species, prairie voles (Microtus ochrogaster) are socially monogamous. Despite this classification, there is considerable individual variation in the propensity to form a partner preference and to display paternal care among male prairie voles. Thus prairie voles are a powerful model species to investigate the neurobiological mechanisms underlying variation in social behavior. The vasopressin V1a receptor (V1aR) plays a central role in both pair bond formation and paternal care in the male prairie vole. Variation in both of these behaviors has been associated with variation in V1aR density in several brain regions, including the lateral septum (LS). The goal of this project is to explore how variation in V1aR density in the LS leads to within-species diversity in male social behavior. First, a siRNA approach will be utilized to knock-down V1aR density in the LS. siRNA injected voles will be tested and compared to controls for their propensity to form a partner preference and to engage in paternal behavior. Second, a selective breeding strategy will be used to determine whether V1aR density in the LS is a major source of heritable variation in social behavior. Once the lines are established, the density of V1aR binding in the LS and other brain regions will be quantified.